[PATCH 1/2] f2fs: fix to avoid potential negative .f_bfree

[Chao Yu]

When calculating .f_bfree value in f2fs_statfs(), sbi->unusable_block_count
can be increased after the judgment condition, result in overflow of
.f_bfree in later calculation. This patch fixes to use a temporary signed
variable to save the calculation result of .f_bfree.

	if (unlikely(buf->f_bfree <= sbi->unusable_block_count))
 		buf->f_bfree = 0;
 	else
		buf->f_bfree -= sbi->unusable_block_count;

Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
---
 fs/f2fs/super.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 2376bb01b5c4..fcc9793dbc2c 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -1216,6 +1216,7 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	u64 id = huge_encode_dev(sb->s_bdev->bd_dev);
 	block_t total_count, user_block_count, start_count;
 	u64 avail_node_count;
+	long long bfree;
 
 	total_count = le64_to_cpu(sbi->raw_super->block_count);
 	user_block_count = sbi->user_block_count;
@@ -1226,10 +1227,12 @@ static int f2fs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	buf->f_blocks = total_count - start_count;
 	buf->f_bfree = user_block_count - valid_user_blocks(sbi) -
 						sbi->current_reserved_blocks;
-	if (unlikely(buf->f_bfree <= sbi->unusable_block_count))
+
+	bfree = buf->f_bfree - sbi->unusable_block_count;
+	if (unlikely(bfree < 0))
 		buf->f_bfree = 0;
 	else
-		buf->f_bfree -= sbi->unusable_block_count;
+		buf->f_bfree = bfree;
 
 	if (buf->f_bfree > F2FS_OPTION(sbi).root_reserved_blocks)
 		buf->f_bavail = buf->f_bfree -
-- 
2.18.0.rc1